1. Field of the Invention
This invention relates to phase shift masks as used in photolithography for fabrication of very large scale integrated circuits. More specifically, this invention relates to a method for forming transition regions between adjacent phase shift mask regions which are of different phase to create a smooth transition between the two phases after optical exposure and hence a transition region in a feature of the integrated circuit with no artifacts due to phase shift intensity nulls.
2. Description of the Prior Art
Phase shifting mask technology for optical lithography is well known. If light from a coherent or partially coherent source is divided by a mask into two or more beams that are superimposed, the intensity in the region of super position varies from point to point between maxima, which exceed the sum of the intensities in the beam, and minima, which may be zero. This phenomenon is called interference. Phase shifting can influence the location and intensity of the interference maxima and minima by adjustment of pattern design and deliberate phase change of the light beams. This results in imaging of higher spatial frequency objects, enhancement of edge contrast, larger exposure latitude, and/or improved depth of focus. Phase shifting is typically accomplished by introducing an extra patterned layer (or layers) of transmissive material on the mask. It is alternatively implemented by etching the mask substrate to various thicknesses so as to effectively provide at least one extra layer. As light propagates through the substrate and through the extra layer, the wave length of the light is reduced from that in the ambient air by the refractive indices of the substrate and the extra layer, respectively.
The optical path difference between the two beams through the extra material layer and without it, is (n-1)a where n is the refractive index of the extra layer and a is the thickness of the extra layer. The phase difference is then proportional to the optical path difference divided by the wave length (in vacuum) of the transmitted light. Usually a phase shift of .pi. (180.degree.) is desirable, i.e., one-half the wave length The added layer is usually considered to be the phase shifter. Further details of various means for implementing of phase shifting are in "Phase Shifting and Other Challenges in Optical Mask Technology," 10th Bay Area Chrome Users Symposium, 1990, by Burn J. Lin, dated Sep. 26, 1990.
Thus, phase shifting is implemented so that the waves transmitted through two adjacent apertures in a photolithography mask are 180.degree. out of phase with one another, whereby destructive interference minimizes the intensity between the images formed by the two apertures. Any optical system will project the images of such a phase shifting transmission object with better resolution and higher contrast than a corresponding transmission object without phase shifts. This improvement in resolution and contrast is highly advantageous in fine line optical lithography, as is typical of fabrication of very large scale integrated circuits.
One problem encountered with use of phase shift masks occurs when two adjacent regions of the mask which abut one another are of different phases. The presence of a transition region between two adjacent regions of an opposite-type phase is typically encountered for instance when a "T" junction is needed or at an "elbow" in which the two arms of the elbow are of different phase, or whenever two linear structures abut one another. For instance, as shown in top view in FIG. 1(a), a mask includes region A of phase 0.degree. and region B of phase 180.degree., where regions A and B are intended to print on a wafer two adjacent structures which together define a single feature, such as a semiconductor region The areas 10, 12 on either side of regions A and B are the unexposed opaque portions of the mask which are covered with a material such as chrome. Note that FIG. 1(a) is a computer generated optical simulation for a projection lithography system using the Hopkins model of partial coherence called "SPLAT" (a well known system from the University of California, Berkeley) as are FIGS. 1(b) and the associated FIGS. 2 to 3. The printed line width of the feature in FIGS. 1 through 3 is 0.35 .mu.m.
The mask shown in FIG. 1(a) (as is understood from optics) forms an intensity null in the image formed on the wafer at the junction between regions A and B as shown in FIG. 1(b). This undesirable intensity null 16 is shown in the inverted areal image intensity (dotted lines) in FIG. 1(b) where instead of the feature line defined by regions A and B being a straight clearly defined line there is instead bridging, i.e., unexposed portions (dotted line) 16 at the transition region. In a typical application this means that a semiconductor region defined by regions A and B might include a highly undesirable discontinuity.
A number of techniques have been proposed to solve this problem. One solution is use of the mask as shown in FIG. 2(a) which includes region A which is 0.degree. phase, region B which is 180.degree. phase, and also intermediate region C which is 90.degree. phase.
It is to be understood that in the phase shifting mask context, typically a 0.degree. phase means that there is no phase shifter layer present over that region. A 180.degree. phase indicates that there is a full thickness of transparent material (phase shifter layer) over that region of the mask. For example the full thickness is about 3600 .ANG. which is one-half of the wavelength divided by (n-1), where n is the index of refraction of the phase shifter (here 1.5) for 365 nm light used to print the mask pattern onto a wafer. A 90.degree. phase indicates that the thickness of the phase shifter layer over that portion of the mask is one-half of the full thickness, i.e., approximately 1800 .ANG. in this example. It is to be understood that the various thicknesses of the phase shift layer are achieved by conventional masking and etching steps of the mask. The phase shifting layer material may be a resist such as PMMA or SOG (spun-on-glass). In another version, the actual mask substrate material (such as quartz) is conventionally masked, exposed by E-beam equipment, then etched to define subregions of various thicknesses, each thickness defining one phase.
The mask shown in FIG. 2(a) (in simulation) when used for integrated circuit fabrication prints the feature on the wafer as shown (also simulated) in FIG. 2(b). The image intensity is again shown by the white lines in FIG. 2(b). In FIG. 2(b) there are still significant intensity nulls 22, 24 at the two junctions defined between regions A and C and between C and B of FIG. 2(a). Again, this is undesirable because it is a degraded image and hence may produce a faulty semiconductor device (i.e., reduced fabrication yield).
Note however that there is a distinct improvement (in terms of the lessening of the intensity nulls) of FIG. 2(b) over that of FIG. 1(b). Thus it is clear that introducing one additional phase, i.e., having the three phases 0.degree., 90.degree. and 180.degree., provides some improvement although this is probably insufficient for very small line width integrated circuits. It is to be appreciated that phase shifting is most useful for small line width integrated circuits, such as those having feature sizes well below 1 micrometer.
FIG. 2(c) is an exposure defocus diagram of the image shown in FIG. 2(b), showing the exposure defocus contours for a given critical dimension (such as the line width). The vertical axis of FIG. 2(c) is an arbitrary scale of exposure intensity; the horizontal axis shows defocus distance in .mu.m. The shaded area is where lithography is acceptable, i.e., the critical dimension is maintained within +10% tolerance. The exposure contours are taken along horizontal lines (labelled a and b) through the line feature of FIG. 2(b). The exposure latitude as defined by the shaded areas is fairly narrow even at 0 .mu.m defocus distance, and falls off sharply at higher defocus distances
A known improvement over the FIG. 2(a) mask is to introduce an additional phase so as to have four phases forming a transition region As shown in FIG. 3(a), here a mask includes region A having a 0.degree. phase, region B having a 180.degree. phase, region D having a 60.degree. phase, and region E having a 120.degree. phase. This provision of four phases provides a smoother transition between the 0.degree. region A and the 180.degree. phase region B in the resulting image and as shown in simulation in FIG. 3(b), will form a feature on the wafer with reduced intensity nulls 28, 30, 32, i.e., closer to a continuous line. However, simulation indicates that even the four phase FIG. 3(b) version exhibits to a certain extent the undesirable intensity nulls 28, 30, 32. FIG. 3(c) shows an exposure defocus diagram for the image of FIG. 3(b). Note that the gray area is significantly broader at both 0 .mu.m and at higher defocus distances than was the correspondence diagram of FIG. 2(c), but still is quite narrow at the higher defocus distances. Here, the exposure contours are taken along four arbitrarily located horizontal lines (a, b, c, d) of the image of FIG. 3(b).
This suggests that five or more phases are needed for excellent resolution at for instance 0.35 .mu.m feature sizes. FIG. 4 shows an exposure defocus diagram for 15 phases (the corresponding mask and image simulations are not shown). While 15 phases provides a broad exposure latitude at 0 .mu.m defocus distance, the latitude (gray area) still narrows significantly at 2 .mu.m defocus distance. Thus even a very large number of phases still results in a less than optimum image using the prior art technique.
However, the solution to this problem as provided by the four phase mask of FIG. 3(a) (or a mask with for instance five phases) has the significant disadvantage that such multiple phase regions substantially increase the cost and complexity of mask making This is because exposing additional phase shifter layers using the usual electron beam mask lithography system complicates mask production, because transfer of the desired pattern to the phase shift medium in the mask increases mask fabrication complexity and time. The increase in complexity requiring extra process steps significantly reduces yield of the masks and hence increases cost.
The mask shown in top view in FIG. 3(a) is shown in a cross-sectional side view in FIG. 5 with the mask formed on a quartz substrate 40. Three thicknesses of phase shifters B, E, D are formed on the substrate. The greatest thickness is in region B which is the 180.degree. phase shift. Regions D and E are comparatively thinner, proportional to the amount of the phase shift. In region A where there is 0.degree. phase shift, no phase shifter material is present. Thus the transition region is a step-type ramp as seen in cross-section. As discussed above, this structure while it approaches the goal of a properly printing transition region, has the significant disadvantage of being relatively difficult and complex to form on the mask and also requires additional processing steps to form up to five phases.
Also relating to phase shifting, U.S. Pat. No. 4,902,899 issued Feb. 20, 1990 to Burn J. Lin et al. is directed to a lithographic process having improved image quality that employs a mask including a plurality of opaque elements or transparent elements smaller than the resolution of the lithography. At column 5, beginning at line 11 Lin et al. disclose use of pixels in a phase shifting mask where different pixels define regions of varying thicknesses, for phase shifting masks. However this document does not disclose any implementation of the described method. Also, this document is not directed to the problem of the junction between two abutting regions of different phase, i.e., the undesirable existence therein of intensity nulls.